Automatic riveting machines are used in connection with fabricating the components of aircraft, for example the wing and fuselage portions of the aircraft. The machines are capable performing tooling operations on a workpiece, for example a panel or stack of panels. The operations typically include drilling and countersinking holes, positioning rivet slugs in the drilled holes, upsetting the rivet slugs to form an enlarged head at one end or a pair of enlarged heads at opposite ends of the slug, and shaving each rivet head such that it is substantially flush with the surface of the workpiece.
In order to perform the various operations necessary to install a rivet, an automatic riveting machine may comprise a head assembly locatable relative to the workpiece at respective rivet installation positions. In one configuration, the head assembly is mounted on a C-frame and includes an upper head and a lower head, and the workpiece is clamped between the upper and lower heads. More specifically, the upper head may include a pressure foot bushing having a clamping end for engaging a surface of a workpiece, a supported end opposite the clamping end, and a passage sized and arranged such that a tool is movable along a tool axis through the pressure foot bushing. The pressure foot bushing may be urged into clamping engagement against a surface of the workpiece by a pressure foot plate coupled to the supported end of the pressure foot bushing and acted upon by one or more actuators.
In another configuration of an automatic riveting machine, the head assembly is an “end effecter” mounted at the end of robot arm. The end effecter comprises a pressure foot bushing and an associated pressure foot plate, whereby a clamping end of the pressure foot bushing may be brought into pressing engagement with a surface of a workpiece.
When the pressure foot bushing is exerting a known force against stacked panels or against a panel on a fixture, any differences in the force required to press the panels together or against the fixture at given fastener locations will change the force imparted on the pressure foot bushing.
In an alternative arrangement, the clamping end of the pressure foot bushing is braced by a fixed pressure foot plate to oppose a predetermined clamping force exerted against an opposite side of the workpiece in a direction toward the clamping end of the pressure foot bushing. If the force required to clamp workpiece panels together or to push a workpiece panel to contact the pressure foot bushing is not the same at each fastener location, the actual force imparted on the pressure foot bushing will vary from fastener location to fastener location.
During operation, automatic riveting machines may experience errors that affect quality, safety, or both quality and safety. One type of error is a workpiece positioning error. The riveting machine is typically set up to run certain fasteners using small test panels (“coupons”) duplicating the thickness and material, but not the total size, of the production panels. The difference in weight and how the test coupon is held compared to the fixtured production panel may introduce positioning errors when running the production panels. Sensors may also be used to control the robotic or CNC positioner of the riveting machine to set panel position in regard to the bottom surface of the pressure foot bushing. The sensors have a certain amount of offset from the actual tooling point of the bushing. If the panel is highly contoured or oddly shaped, the offset of the sensors can change the panel position. The position of the pressure foot bushing side of the panel is very important for the fastening process. For example, sometimes a countersink is drilled in the pressure foot side of the panel. The depth of the countersink will vary if panel position differs from one fastener location to the next. As another example, if there is error in the panel position, the anvil will not be in the correct position to contact the fastener which can create panel distortion during the fastening process. The anvil will also not seat an interference fit fastener properly if the panel is not in the correct position. Sometimes a fastener has its head on the pressure foot side shaved flush; again, any change in panel position will affect the shaved height of the fastener. Positioning errors of the kinds mentioned above often result in poor quality in the end product, and can lead to scrapping of expensive panels. In extreme cases, operator injury may result.
Another type of error is insufficient clamping force. This can lead to dangerous conditions. For example, when running set up operations on coupons, an unclamped or under-clamped coupon can spin during the drilling operation and potentially injure personnel.
It is also known to have errors in the automatic fastening process related to other factors. For example, a foreign object may become lodged between the pressure foot bushing and the panel surface. As another example, the drill bit used for drilling operations or the cutter used for shaving operations may become dull or break, and this will have a negative impact on quality.
Heretofore, manual measurement and operator observation have been used to check for errors during fastening program execution. Manual measurement slows down execution, and the operator observation is not always reliable. Often, program execution is halted after it is too late to save the panel or prevent injury. Thus, there is a need for an improved apparatus and method capable of automatically detecting unintended machining conditions at an early stage.